Menu:

Version:

5-digit Frequency Counter and Crystal Tester

- Review and 3D Printed Enclosure

Here
is a review of a simple kit that is easy to build and works well. I
also provide details of a 3D printed box which provides a tidy finish
to the project.

Introduction

In the
past, whenever I have needed to test a crystal, I have often quickly
built a single-transistor oscillator and measured the result with a
frequency counter. Once the test was done, the parts disappeared back
into the parts box until next time. Then I’d have to build another one.

Several
months ago, I came across an online video review of a widely available
crystal tester/frequency counter kit. It was not expensive, about $US6
including postage. At that price, it looked like a nice solution to my
usual temporary "build it when you need it" approach. So, I ordered one.

The
kit arrived promptly from the one of my regular Chinese vendors. I
recognised the circuit almost immediately - It was a slightly modified
version of the frequency counter designed by DL4YHF. You can find some
outline details of the frequency counter section on his website. I
couldn't find any mention or description anywhere about the crystal
tester part of the kit, so I decided to note some of the details here.

Kit Construction

My
overall impression of the kit is very positive. I checked the parts
first, and nothing was missing. The components were all good
quality parts. The PCB is well made, clearly marked, and parts are well spaced on the board. The
kit was didn’t take long to assemble. It’s very easy to build, although
I would not recommend it for the absolute beginner. I took my time,
taking several (essential) coffee breaks. It took just a couple of hours to put it together.

Before
inserting the PIC microcontroller, I briefly powered it up with my
bench variable power supply to check that the supply voltages on
the board were correct. With this done, I then inserted the PIC and
turned the device on.

Initial Testing

I
tested the board with as many crystals as I could find. Those tests
confirmed that the tester worked accurately with all sorts of crystals
ranging from 1 to 50MHz. Some higher frequency “overtone” crystals
oscillated on their fundamental frequency, as expected. For example, a
27MHz crystal oscillated at 9MHz. This is quite normal - The design for
an overtone crystal oscillator is quite different to the circuit used
in the wide range crystal test oscillator in the crystal tester.

Several
32.768kHz and 38kHz watch-type crystals and ceramic resonators did not
work with the tester, and that was as I expected. Just like the
overtone crystals, these low frequency crystals and ceramic resonators
require completely different oscillator designs to work correctly. (I
may build a little add-on module with a couple of different types of
test oscillators to provide those functions in future)

Figure 1 : 20MHz crystal being tested

The
most important result from my testing of the tester/counter was that
all of my standard crystals worked in the tester without any problems
at all.

I proceeded to test the frequency counter. It was
able to count one of my signal generators right up to 50MHz with ease.
I didn’t measure its sensitivity. The counter section of thie tester
doesn't have any type of preamplifier/buffer stage so it will only
count relatively large signals, say around 1-3Vpp. Other pages
here on my website show the details for some simple counter preamps
that could be added if you wish to add a preamp to this counter. One
example is here.

I also
tested it with several power supplies including a standard 9V battery,
a 6V wall plug pack power supply, and a tiny 5V USB-type celphone
charger. They all powered the crystal checker without any problems. The
LED display was a little less bright when using the 5V USB supply, but
it still was perfectly usable.

To calibrate the frequency
counter, I inserted a known good crystal into the tester and measured
the output of the crystal tester’s oscillator with one of my frequency
counters. The on-board trimmer capacitor was then adjusted very
slightly to set it exactly on the same frequency as my counter.

There
are some other special features available through the ‘Program’ button
including automatic offset programming, but I did not test these
functions. More details are found in the kit instructions and on
DL4YHF’s website.

Crystal Tester Schematic

Unfortunately,
the kit doesn’t include an accurate schematic, or even a schematic
showing all of the sections of the device and how they interconnect.
The details in the kit are taken directly from one of the drawings
on DL4YHF’s website, and they only outline a small section of the
frequency counter part of the tester. Frankly, even those details are
unclear. There were no details of the crystal oscillator, the power
supply regulation circuit, or the external connections.

Faced with that, I promptly spent an hour drawing up the complete schematic. It’s shown below:

Figure
1 : Schematic of the crystal checker - This shows the counter, the test
oscillator and the power supply (Right-click on this drawing to see the
full scale version)

Circuit Description

The
schematic reveals a fairly typical PIC-based frequency counter circuit.
It uses one of the counter/timer inputs to count the crystal
oscillator or an external signal which can be connected to the
counter input via J3. The measured frequency is then displayed on the
five multiplexed seven-segment LEDs.

The crystal
oscillator is a standard Colpitts oscillator. Q1 provides the gain, and
oscillator feedback is via C3 and C4. The oscillator’s output is taken
from the emitter of Q1 via C5 which then goes to the PIC
microcontroller. This same connection is shared with the external
frequency input. This means that the crystal oscillator will load any
external circuit which is connected to the external input (J2) so some
care may be required in some cases.

The external power
supply is connected to J3. It is regulated by a low voltage drop
HT7550-1 regulator (IC2). This is a good choice since the low voltage
drop means the checker will work with an external 5V power supply like
the USB charger I tried.

There is one potential problem
revealed by the schematic. The crystal oscillator transistor is powered
directly from the external power supply. If the crystal tester is
powered, say, by a 9V battery, then the oscillator output, which
is connected directly to the PIC, will exceed the maximum limits of the
PIC. I measured this to check. With a 9V battery, the input signal on
the PIC’s pin 3 was a sine wave measuring 8V peak to peak!

There
is no simple fix for this. The 1n capacitor (C5) could be reduced in
size, or Q1 could be connected directly to the output of the 5V
regulator, or a resistor divider could be added to the input of the PIC
chip. However, all of these options will result in reduced
performance.

A better solution would be to redesign the
oscillator to use, say, a 74HC04 as the test oscillator, with the
74HC04 supplied from the regulated 5V rail. Spare gates could be then
be used for oscillators for those other types of crystals I mentioned
earlier, such as the 32 and 38kHz crystals. Another 74HC04 gate could
be used for a ceramic resonator oscillator, and a further gate (There
are six in the 74HC04 IC) could be dedicated for use as a counter
preamp, for buffering and counting external inputs.

Well,
I gave that idea about 30 seconds of thought before deciding to simply
ignore the problem. Given the price of the kit and the typically very
short time the oscillator is used during each test, it’s a reasonable
compromise. "Ignorance is bliss", as the saying goes.

Finishing Touches

I
searched the usual websites for a suitable 3D-printed box for the
crystal tester but I couldn't find anything. That was easy to fix. It
only took a couple of hours for me to design and print a suitable
enclosure. I used PLA filament as usual, printed using 0.2mm layers and
20% fill. Blue PLA was on the printer when I went to print the box, so
that’s what I used.

Figure
2 : My 3D printed case includes space for a 9V battery and a mounting
slot for a standard slider power switch. The battery, battery connector
and switch need to be purchased separately.

I
designed the enclosure with space for a standard 9V battery. This makes
it completely portable. A battery is fine for this sort of instrument
where it is only likely to be used for a few seconds perhaps once or
twice a week, at most. The battery (and its clip-on connector) fit in
the slightly larger right hand end of the enclosure. The crystal tester
PCB fits inside the cover and base. M3 self-tapping screws about 10mm
long hold everything together.

All told, the finished box
measures 110mm wide and 60mm deep. The left hand side is just under 18
mm high while the battery end is 21 mm high. You can see the hole added
on the left hand end of the enclosure for a slide switch to turn the
power on and off.

I also designed a simple front panel for
the case. I printed it onto plain paper using a colour laser printer
and covered it with clear self-adhesive plastic. It was then glued to
the printed enclosure.

Figure 3 : Front panel for the 3D printed box. Crystals can be plugged into the socket on the left (blue arrows)

The
grey shaded boxes on the panel artwork should be removed with a sharp
knife to expose the display and the two connectors. The grey circle
should be cut out to allow the on-board pushbutton switch to be pressed
when those functions are required.

The little blue arrows on
the panel artwork indicate the pins used to connect the crystal being
tested. The red and green arrows indicate the external input pins for
the frequency counter.

The standard STL files for the 3D case and a JPG file for the front panel artwork are available for downloading below.